Synthesis and Electronic Properties of Intensely Coloured Iron(II) complexes with new 4‐substituted planar tridentate nitrogen ligands analogous to 2, 2′: 6′, 2″‐terpyridine

The para ‐substituted tridentate ligands 2,6‐bis[1‐(3,5‐dimethoxybenzyl)benzimidazol‐2‐yl]‐4′‐X‐phenyl)‐pyridine (X = H: L 3 ; X = NO 2 : L 4 ; X = NEt 2 : L 5 ) have been synthesized in six steps using Kröhnke reactions. Spectroscopic results show that the electronic properties of the ligands are strongly influenced by the nature of the substituent bound to the pyridine ring and extended Hückel calculations performed on 4‐(X‐phenyl)pyridine fragments give a qualitative insight into the factors responsible for the striking differences observed in the absorption spectra. Upon complexation to Fe II , the ligands L 2 –L 5 give intensely coloured pseudo‐ocatahedral low spin [Fe(L n ) 2 ] 2+ (n = 2–5) complexes in acetonitrile solution. Electrochemical and spectroscopic measurements show that the particular electronic characteristics associated with each substituent strongly influence the properties of the resulting Fe II complexes as demonstrated by the very intense MLCT transition (ϵ > 50000 M −1 cm −1 ) observed in the green complex [Fe(L 5 ) 2 ] 2+ .